A housing of a small-sized motor is configured such that a casing lid made of resin is fitted to an opening portion of a motor casing which is formed from metal into a closed-bottomed tubular shape. Patent Document 1 discloses an example of a conventional structure for fixing together a motor casing made of metal and a casing lid made of resin.
FIG. 8 is a view showing an example of a conventional fixing structure (refer to FIG. 1(c) of Patent Document 1). As illustrated, after the casing lid made of resin is fitted to an opening portion of the motor casing made of metal, an opening end portion of the motor casing is bent at several positions (bend portions in FIG. 8) by use of a punch, and the bend portions are pressed against (brought into close contact with) the casing lid for fixation. In this manner, by means of bending the opening end portion of the motor casing, the casing lid can be fixed in a sufficiently firm manner. However, in the case of a casing lid having a large volume and a complicated shape, the casing lid is susceptible to dimensional changes under the influence of temperature and humidity. Particularly, upon occurrence of a dimensional change of the casing lid toward the center of the casing lid, clearance is generated at fixed portions, resulting in the occurrence of a defect of loosening of fixation between the motor casing and the casing lid. When, in order to restrain the occurrence of loosening, the bend portions of the motor casing are increased in length and bent deeper, deformation or breakage of the casing lid may result.
FIG. 9 is a view showing another example of a conventional fixing structure (refer to FIG. 4(b) of Patent Document 1). An opening end portion of the motor casing is bent at several positions (pawls in FIG. 9) by use of a punch; however, the pawls are not pressed against (not brought into close contact with) the casing lid, but adhesive is charged between the pawls and the casing lid for fixation. By means of fixing together the casing lid and the bent pawls by use of adhesive, the casing lid can be fixed in a sufficiently firm manner. However, because of differences in thermal expansion coefficient among the metal used to form the motor casing, the resin used to form the casing lid, and the adhesive, separation arises at the bonded positions under the influence of temperature and humidity unless the metal, the resin, and the adhesive are fused together; thus, loosening of fixation between the motor casing and the casing lid arises. Also, use of adhesive increases cost.    Patent Document 1: Japanese Utility Model Application Laid-Open (kokai) No. S56-30553.